Aqueous dispersion of multistage polymer particles

ABSTRACT

The present invention relates to a composition comprising an aqueous dispersion of multistage polymer particles comprising a first and a second phase, wherein the first phase comprises structural units of a carboxylic acid monomer or a salt thereof, and a nonionic ethylenically unsaturated monomer; and wherein the second phase comprises a first and second polymer, wherein the first phase or the second phase first polymer or both comprise structural units of a high T g  hydrophobic monomer; and wherein the second phase second polymer comprises at least 80 percent structural units of styrene. The high T g  hydrophobic monomer is defined as being one or more of the following monomers: cyclohexyl methacrylate, isobornyl methacrylate, 4-t-butyl methacrylate, t-butylstyrene, or n-butyl methacrylate. The multistage polymer particles are useful as opaque polymers, which are used in pigmented coating formulations to reduce the load of TiO 2 . The particles exhibit excellent collapse resistance and unusually low dry bulk density, and do not require acrylonitrile to achieve this desired combination of properties.

BACKGROUND OF THE INVENTION

The present invention relates to an aqueous dispersion of multistagepolymer particles that is useful for improving hiding efficiency in apigmented coatings formulation.

Titanium dioxide (TiO₂) is the opacifying pigment of choice for use inpaint formulations due to its exceptionally high refractive index;however, the high cost of TiO₂ has motivated researchers to investigateways to reduce its loading while maintaining high opacifying (hiding)efficiency. One such approach has been the development andcommercialization of high scattering polymeric pigments known as opaquepolymers, which have been found to preserve hiding efficiency at a lowerpigment volume concentration (PVC) TiO₂. U.S. Pat. No. 6,020,435discloses the preparation of an aqueous dispersion of core-shell polymerparticles containing acid-functionalized cores, which are converted toopaque polymers upon neutralization of the core and subsequently coatinga substrate with the dispersion, thereby allowing water to evaporate toform a film with voided particles.

The efficiency improvement from opaque polymers arises primarily fromtwo factors: Low dry bulk density and collapse resistance;unfortunately, efforts to achieve lower dry bulk density to achievefurther improvements in hiding efficiency reduces resistance tocollapse. This correlation is unsurprising because low dry bulkdensities correlate with a larger core, therefore a largerweight-to-weight core-to-shell ratio at the desired particle size; theresult is a thinner shell that is more susceptible to collapse. It hasbeen found that acrylonitrile incorporation into an intermediate orpost-intermediate polymerization stage—the stage or stages following thecore stage—results in the formation of opaque polymers with lower drybulk density and at an acceptable collapse resistance; nevertheless,acrylonitrile is acutely toxic and has been observed to cause mucousmembrane irritation, headaches, dizziness, and nausea to exposedworkers. Therefore, it would be advantageous to prepare collapseresistant low dry bulk density polymer particles without acrylonitrilefunctionalization.

SUMMARY OF THE INVENTION

The present invention addresses a need in the art by providing acomposition comprising an aqueous dispersion of multistage polymerparticles comprising a first and a second phase, wherein:

a) the first phase comprises, based on the weight of the first phase,from 25 to 60 weight percent structural units of a carboxylic acidmonomer or a salt thereof, from 40 to 75 weight percent structural unitsof a nonionic ethylenically unsaturated monomer, and up to a total of 15weight percent structural units of styrene and a high T_(g) hydrophobicmonomer; and wherein

b) the second phase comprises a mixture of first and second polymers,wherein

-   -   1) the first polymer comprises, based on the weight of the first        polymer, i) from 5 to 15 weight percent structural units of a        carboxylic acid monomer or a salt thereof; ii) from 45 to 55        weight percent structural units of styrene; and iii) from 37 to        47 weight percent structural units of methyl methacrylate or the        high T_(g) hydrophobic monomer or a combination thereof; and    -   2) the second polymer comprises, based on the weight of the        second polymer, i) from 80 to 99.9 weight percent structural        units of styrene; and ii) from 0.1 to 0.5 weight percent        structural units of a multiethylenically unsaturated monomer;

wherein the weight-to-weight ratio of the first phase to the secondphase is in the range of from 1:9 to 1:20; and wherein theweight-to-weight ratio of the first polymer of the second phase to thesecond polymer of the second phase is in the range of from 1:3 to 1:8;

with the proviso that the first polymer and the first phase of thesecond polymer together comprise, based on the weight of the firstpolymer and the first phase of the second polymer, from 2 to 15 weightpercent structural units of the high T_(g) hydrophobic monomer, which isone or more monomers selected from the group consisting of cyclohexylmethacrylate, isobornyl methacrylate, 4-t-butyl methacrylate,t-butylstyrene, and n-butyl methacrylate. The composition of the presentinvention addresses a need by providing opaque polymers with excellentcollapse resistance and unusually low dry bulk density, withoutacrylonitrile.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a composition comprising an aqueous dispersionof multistage polymer particles comprising a first and a second phase,wherein:

a) the first phase comprises, based on the weight of the first phase,from 25 to 60 weight percent structural units of a carboxylic acidmonomer or a salt thereof, from 40 to 75 weight percent structural unitsof a nonionic ethylenically unsaturated monomer, and up to a total of 15weight percent structural units of styrene and a high T_(g) hydrophobicmonomer; and wherein

b) the second phase comprises a mixture of first and second polymers,wherein

-   -   1) the first polymer comprises, based on the weight of the first        polymer, i) from 5 to 15 weight percent structural units of a        carboxylic acid monomer or a salt thereof; ii) from 45 to 55        weight percent structural units of styrene; and iii) from 37 to        47 weight percent structural units of methyl methacrylate or the        high T_(g) hydrophobic monomer or a combination thereof; and    -   2) the second polymer comprises, based on the weight of the        second polymer, i) from 80 to 99.9 weight percent structural        units of styrene; and ii) from 0.1 to 0.5 weight percent        structural units of a multiethylenically unsaturated monomer;

wherein the weight-to-weight ratio of the first phase to the secondphase is in the range of from 1:9 to 1:20; and wherein theweight-to-weight ratio of the first polymer of the second phase to thesecond polymer of the second phase is in the range of from 1:3 to 1:8;

with the proviso that the first polymer and the first phase of thesecond polymer together comprise, based on the weight of the firstpolymer and the first phase of the second polymer, from 2 to 15 weightpercent structural units of the high T_(g) hydrophobic monomer, which isone or more monomers selected from the group consisting of cyclohexylmethacrylate, isobornyl methacrylate, 4-t-butyl methacrylate,t-butylstyrene, and n-butyl methacrylate.

The multistage polymer particles of the present invention preferablyhave a core-shell morphology wherein the first phase corresponds to thecore and the second phase corresponds to the shell. The core may beproduced by a single stage or a multistage process, preferably in thepresence of a chain transfer agent such as n-dodecyl mercaptan ormercaptoethanol. The core may also be prepared from a seed process. Apreferred method of preparing the core is described in U.S. Pat. No.6,020,435.

Preferably, the first phase comprises from 30, more preferably from 35,and most preferably from 38 weight percent, to preferably 50, morepreferably to 45, and most preferably to 42 weight percent structuralunits of a salt of a carboxylic acid monomer, based on the weight of thefirst phase. As used herein, the term “structural units” refers to theremnant of the recited monomer after polymerization. For example, astructural unit of a salt of methacrylic acid, where M⁺ is a counterion,preferably a lithium, sodium, or potassium counterion, is asillustrated:

The first phase also preferably comprises from 50, more preferably from55, and most preferably from 58 weight percent, to preferably 70, morepreferably to 65, and most preferably to 62 weight percent structuralunits of a nonionic ethylenically unsaturated monomer.

The first phase preferably comprises from 5 to 10 weight percent totalstructural units of styrene and/or a high T_(g) hydrophobic monomerselected from the group consisting of cyclohexyl methacrylate, isobornylmethacrylate, 4-t-butyl methacrylate, t-butylstyrene, and n-butylmethacrylate. The term “high T_(g) monomer” refers to a monomer thatforms a homopolymer that is not film-forming at 25° C. In one preferredembodiment, the first phase comprises from 5, more preferably from 6weight percent to 10 weight percent structural units of the high T_(g)hydrophobic monomer, based on the weight of the first phase. In anotherembodiment, the first phase comprises, based on the weight of the firstphase, less than 10, more preferably less than 5, and most preferablyless than 1 weight percent structural units of styrene.

Examples of carboxylic acid functionalized monomers include methacrylicacid, acrylic acid, and itaconic acid, with acrylic acid and methacrylicacid being preferred. Examples of nonionic ethylenically unsaturatedmonomers include C₁-C₁₀ alkyl acrylates and methacrylates such as methylmethacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, butylmethacrylate, and 2-ethylhexyl acrylate; and styrene. Methylmethacrylate and butyl methacrylate are preferred nonionic ethylenicallyunsaturated monomers used to prepare the first phase.

The average particle size of the first phase is preferably in the rangeof from 80 nm to 150 nm as measured by light scattering using a BI-90Plus Brookhaven Particle Analyzer.

The second phase first polymer preferably comprises from 6, morepreferably from 7 weight percent, to 12, more preferably to 9 weightpercent structural units of a salt of a carboxylic acid monomer,preferably an alkali metal salt of methacrylic acid of acrylic acid. Thesecond phase second polymer preferably comprises from 83 to 91 weightpercent structural units of styrene; preferably from 8 to 12 weightpercent structural units of methyl methacrylate; preferably from 0.5 to4 weight percent structural units of a salt of a carboxylic acidmonomer, preferably an alkali metal salt of methacrylic acid or acrylicacid; and preferably from 0.1 to 0.4 weight percent structural units ofa multiethylenically unsaturated monomer such as divinyl benzene orallyl methacrylate. In another embodiment, the second phase firstpolymer comprises from 5 to 15 weight percent structural units of thehigh T_(g) hydrophobic monomer, based on the weight of the second phasefirst polymer. In another embodiment, the second phase first polymercomprises, based on the weight of the second phase first polymer, lessthan 10, more preferably less than 5, and most preferably less than 1weight percent structural units of styrene.

The weight-to-weight ratio of the first phase to the second phase ispreferably in the range of from 1:11, more preferably from 1:12, to1:18, and more preferably to 1:16; and the weight-to-weight ratio of thefirst polymer of the second phase to the second polymer of the secondphase is preferably in the range of from 1:4, more preferably from 1:5,to preferably 1:7.

The multistage polymer particles are preferably prepared in three stagesfrom the aqueous dispersion of the first phase polymer particles. In apreferred method of preparing the aqueous dispersion of multistagepolymer particles, a first monomer emulsion (ME 1) comprisingmethacrylic acid, methyl methacrylate, and styrene are contacted in akettle with an aqueous dispersion of first phase (core) polymerparticles having a solids content in the range of from 20, morepreferably from 25, to 40, more preferably to 35 weight percent, andcopolymerized under emulsion polymerization conditions to form adispersion of core/tie-layer polymer particles. In one embodiment, thecore polymer particles comprise from 5, more preferably from 6 weightpercent, to 15, and more preferably to 10 weight percent structuralunits of the hydrophobic high T_(g) monomer. ME 1 preferably comprisesfrom 6, more preferably from 7 weight percent, to 12, and morepreferably to 9 weight percent of methacrylic acid or acrylic acid basedon the weight of ME 1 monomers; preferably from 48 to 52 weight percentstyrene based on the weight of ME 1 monomers; and preferably from 20 to40 weight percent methyl methacrylate, based on the weight of ME 1monomers. In one embodiment, ME 1 comprises 0 weight percent of thehydrophobic high T_(g) monomer; in another embodiment, ME 1 comprisesfrom 2, and more preferably from 4 weight percent, to 12, and morepreferably to 10 weight percent of the hydrophobic high T_(g) monomerbased on the weight of ME 1 monomers. The core polymer particles or thetie-layer or both comprise structural units of a high T_(g) hydrophobicmonomer. The weight-to-weight ratio of ME 1 to core is in preferably therange of from 0.5:1, more preferably from 1:1, to 4:1, and morepreferably to 3:1.

The second phase second polymer may be prepared in a single stage but isadvantageously prepared in two stages (designated as ME 2 and ME 3) asfollows: Upon completion of addition of ME 1 to the kettle, a secondmonomer emulsion (ME 2) comprising from 78 to 86.5 weight percentstyrene, from 12 to 18.5 weight percent methyl methacrylate, from 1 to 4weight percent methacrylic acid or acrylic acid, and from 0.1 weightpercent to 0.6 weight percent allyl methacrylate of divinyl benzene isadded to the kettle under emulsion polymerization conditions. ME 2 alsopreferably includes from 0.2 to 0.8 weight percent of linseed oil fattyacid. The weight-to-weight ratio of ME 2 to ME 1 is preferably in therange of from 3.5:1, more preferably from 4:1, and most preferably from4.5:1, to 6:1, more preferably to 5.5:1, and most preferably to 5:1.

After a suitable hold period of ˜15 minutes, a third monomer emulsion(ME 3), which contains styrene and 4-hydroxy TEMPO, is fed into thereactor followed by addition of hot deionized water and a neutralizingamount of a base such as NH₄OH or an alkali metal hydroxide such asconcentrated NaOH. The dispersion is advantageously chased with t-butylhydroperoxide (t-BHP) and isoascorbic acid (IAA) and the contents werefiltered to remove any coagulum. The weight-to-weight ratio of ME 3 toME 2 is preferably in the range of from 0.1:1, more preferably from0.15:1 to 0.5:1, more preferably to 0.3:1, and most preferably to0.25:1.

Accordingly, in another aspect of the present invention, the secondphase second polymer is a mixture of polymers, one of which (secondphase second polymer A) comprises, based on the weight of the secondphase second polymer A, from 78 to 86.5 weight percent structural unitsof styrene, from 12 to 18.5 weight percent structural units of methylmethacrylate, from 1 to 4 weight percent structural units of methacrylicacid or acrylic acid, and from 0.1 weight percent to 0.6 weight percentstructural units of a multiethylenically unsaturated monomer; and asecond of which (second phase second polymer B) comprises, based on theweight of second phase second polymer B, preferably at least 98 weightpercent, preferably 100 weight percent structural units of styrene,wherein the weight-to-weight ratio of second phase second polymer A tosecond phase second polymer B is in the range of from 3:0:1, morepreferably from 3.5:1, more preferably from 4:1, and most preferablyfrom 4.5:1, to 6:1, more preferably to 5.5:1, and most preferably to5:1.

Preferably, the average particle size of the neutralized multistagepolymer particles as measured by light scattering using a BI-90 PlusBrookhaven Particle Analyzer is in the range of from 150 nm, morepreferably from 200 nm, most preferably from 350 nm; to 600 nm, morepreferably to 500 nm, most preferably to 450 nm. The solids content ofthe aqueous dispersion of multistage polymer particles is preferably inthe range of from 10 to 35 weight percent.

The aqueous dispersion of multistage polymer particles is useful as anopacifying polymeric additive that allows for the reduced loading ofTiO₂ in paint formulations. When formulations containing theseopacifying polymer additives are applied as a coating to a substrate andallowed to dry, collapse resistant opaque polymers with a dry bulkdensity in the range of 0.50 to 0.55 g/cc were formed. Collapseresistant opaque polymers with dry bulk densities at this low a levelhave until now only been achieved with inclusion of acrylonitrile in thesecond phase; however, it has been surprisingly discovered thatacrylonitrile, as well as methacrylonitrile, acrylamide, andmethacrylamide, are no longer necessary to achieve the propertiesheretofore only achievable with the inclusion of these difficult tohandle monomers. Accordingly, the second phase of the multistage polymerparticles preferably comprises less than 10 weight percent, morepreferably less than 1 weight percent, more preferably less than 0.1weight percent, and most preferably 0 structural units of acrylonitrile,methacrylonitrile, acrylamide, and methacrylamide, based on the weightof the second phase.

The aqueous dispersion of multistage polymer particles of the presentinvention is useful as a supplementary opacifying pigment in paintformulations. In another aspect, the present invention is a pigmentedwater-based coatings composition comprising a dispersion of themultistage polymer particles, a rheology modifier, a binder, TiO₂, andat least one additive selected from the groups consisting ofsurfactants, defoamers, biocides, dispersants, coalescents, andneutralizing agents.

S/Mil Measurements

Kubelka-Munk Scattering Coefficient (S/Mil)

The scattering coefficient (S/Mil) is a measure of the opacity of theaqueous dispersion of multistage polymer particles. A sample of thedispersion was blended with RHOPLEX™ AC-264 Emulsion Polymer (AC-264, ATrademark of The Dow Chemical Company or Its Affiliates) on a solidsbasis at a weight-to-weight ratio of 15% aqueous dispersion/85% AC-264.A 7-mil wet film of the blend is drawn over a sheet of black vinyl thatwas measured for thickness in four small defined areas with an AmesGauge. The film was dried for 2 h at low relative humidity (<40% R.H.).The reflectance of the dry film was measured by a Gardner InstrumentReflectometer over the four defined areas. The thickness of the driedfilm was also determined over the same defined areas using the AmesGauge. The Scattering coefficient was calculated for each of definedareas as:

${S\text{/}{Mil}} = \frac{R}{\left( {1 - R} \right) \times T}$R = Reflectance T = film  thickness  in  mils

The four S/Mil measurements were then averaged to obtain the S/Mil forthe film.

Collapse

Collapse is an indication of the ability of disperse multistage polymerparticles to resist the forces of drying acting on the walls of theinternal microvoid. These forces are at their greatest when the humidityis high, which causes the particles to dry slowly. Collapse isdetermined using essentially the same procedure that is used indetermining S/Mil above except that a second drawdown is dried overnightat 85% R.H., then dried at <40% R.H. for 1 h.

${\% \mspace{14mu} {Collapse}} = {1 - {\left( \frac{{High}\mspace{14mu} {humidity}\mspace{14mu} S\text{/}{mil}}{{Low}\mspace{14mu} {humidity}\mspace{14mu} S\text{/}{mil}} \right) \times 100}}$

EXAMPLES

The preparation of the aqueous dispersion of the core was carried out asdescribed in U.S. Pat. No. 6,020,435. Table 1 illustrates the monomersand relative amounts used to prepare the aqueous dispersions of thecores, as well as the solids content and average particle size of thecore particles, as measured by a Brookhaven BI 90 particle sizeanalyzer. MMA refers to methyl methacrylate; MAA refers to methacrylicacid; CHMA refers to cyclohexyl methacrylate; t-BuSty refers tot-butylstyrene; Sty refers to styrene; t-BuMA refers to t-butylmethacrylate; t-BuMA refers to t-butyl methacrylate.

Also, the term tie-coat is used to describe the polymers formed frommonomers in ME 1.

TABLE 1 Monomer Distribution, Particle Size, and Solids Content of CoresIntermediate Ex # Monomer Distribution Particle Size % Solids 1 66MMA/34 MAA 140 nm 32.1% 2 62 MMA/34 MAA/4 CHMA 140 nm 32.1% 3 58 MMA/34MAA/8 CHMA 138 nm 32.1% 4 56 MMA/34 MAA/10 t-BuSty 141 nm 32.4% 5 56MMA/10 Sty/34 MAA 135 nm 31.9% 6 56 MMA/10 t-BuMA/34 MAA 140 nm 31.6% 755 MMA/10 BMA/35 MAA 140 nm 31.7% 8 56 MMA/8 IBOMA/34 MAA 140 nm 31.7%

Comparative Example 1—Preparation of Aqueous Dispersion of PolymerParticles with No Hydrophobic High T_(g) Monomer in Core or ME 1

Deionized water (800 g) was added to a 5-L 4-necked round bottom flask(kettle) equipped with a paddle stirrer, thermometer, nitrogen inlet,and reflux condenser; the kettle was heated to 89° C. under N₂ at whichtime sodium persulfate (3.2 g) dissolved deionized water (30 g) wasadded, followed immediately the addition the core of IntermediateExample 1 (186.9 g). A monomer emulsion (ME 1), which was prepared bymixing deionized water (60.0 g), sodium dodecyl sulfate (SDS, 4.0 g, 23%active), styrene (60.0 g), MMA (50.4 g), and MAA (9.6 g) was added tothe kettle at a rate of 3.0 g/min at a temperature of 77-79° C. Uponcompletion of ME 1 addition, a second monomer emulsion (ME 2), which wasprepared by mixing deionized water (187.0 g), SDS (8.0 g, 23% active),styrene (491.4 g), MMA (72.0 g), MAA (10.8 g), linseed oil fatty acid(LOFA 3.6 g), and allyl methacrylate (ALMA 1.80 g), was fed to thereactor at a rate of 10 g/m for 15 min during which time the temperaturewas allowed to rise to 84° C. After 15 min, the feed rate of ME 2 wasincreased to 20 g/min and a separate mixture of sodium persulfate (0.75g) dissolved in deionized water (62.0 g) was co-fed to the reactor at arate of 1.5 g/min. The temperature of the reaction mixture was allowedto increase to 92-93° C. during the course of the ME 2 feed. Uponcompletion of addition of ME 2 and the co-feed, a mixture of 0.1%FeSO4.7H₂O (20.0 g)/1% VERSENE™ Chelating Agent (2.0 g, A Trademark ofThe Dow Chemical Company or its Affiliates) was added to the kettle; thetemperature was held for 15 min at ˜92° C., after which time a thirdmonomer emulsion (ME 3), which was prepared by mixing deionized water(46 g), SDS (1.7 g), styrene (144.0 g), and4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (4-hydroxy TEMPO, 3.0 g),was fed to the kettle at a rate of 40 g/min. After completion of ME 3addition, hot deionized water (300 g) was added to the kettle followedby addition of a mixture of 50% sodium hydroxide (26.6 g) and hot water(450 g) over 10 min. The reaction mixture was then held for 5 min at atemperature of 80-85° C., after which time a mixture of t-BHP (1.2 g)and deionized water (25 g) was added to the kettle. A mixture ofisoascorbic acid (IAA, 0.65 g) and deionized water (50 g) was then fedto the kettle over 25 min. Upon completion of the IAA co-feed, thekettle was cooled to room temperature and the contents filtered toremove any coagulum formed. The final latex had a solids content of28.5%, a pH of 8.6, and a particle size of 427 nm. The dry density ofthis polymer was calculated to be 0.517 g/mL. Low RH S/Mil was 1.34,High RH S/Mil was 0.97, and % Collapse was 28%.

Example 1—Preparation of Aqueous Dispersion of Polymer Particles withCHMA in ME 1

The procedure of Comparative Example 1 was carried out except thatIntermediate Example 1 core (188.1 g) was used; and ME 1 monomers werestyrene (60.0 g), MMA (44.4 g), MAA (9.6 g), and CHMA (6.0 g). The finallatex had a solids content of 28.5%, a pH of 8.9, and a particle size of384 nm. The dry density of this polymer was calculated to be 0.533 g/cc.Low RH S/Mil was 1.31, High RH S/Mil was 1.23, and % Collapse was 6%.

Example 2—Preparation of Aqueous Dispersion of Polymer Particles withCHMA in ME 1

The procedure of Comparative Example 1 was followed except thatIntermediate Example 1 core (188.7 g) was used; and ME 1 monomers werestyrene (60.0 g), MMA (26.4 g), MAA (9.6 g), and CHMA (24.0 g). Thefinal latex had a solids content of 28.8%, a pH of 8.9, and a particlesize of 415 nm. The dry density of this polymer was calculated to be0.541 g/cc. Low RH S/Mil was 1.28, High RH S/Mil was 1.22, and %Collapse was 5%.

Example 3—Preparation of Aqueous Dispersion of Polymer Particles withCHMA in the Core

The procedure of Comparative Example 1 was carried out except thatIntermediate Example 3 core (186.9 g) was used; and ME 1 monomers werestyrene (60.0 g), MMA (26.4 g), MAA (9.6 g), and CHMA (12.0 g). Thefinal latex had a solids content of 28.2%, a pH of 8.9, and a particlesize of 412 nm. The dry density of this polymer was calculated to be0.541 g/cc. Low RH S/Mil was 1.32, High RH S/Mil was 1.32, and %Collapse was 0%.

Example 4—Preparation of Aqueous Dispersion of Polymer Particles witht-Butyl Styrene in the Core

The procedure of Comparative Example 1 was followed except that the core(185.2 g) was made as described in Intermediate Example 4. The finallatex had a solids content of 28.4%, a pH of 8.75, and a particle sizeof 409 nm. The dry density of this polymer was calculated to be 0.541g/cc. Low RH S/Mil was 1.40, High RH S/Mil was 1.28, and % Collapse was8.5%.

Example 5—Preparation of Aqueous Dispersion of Polymer Particles witht-Butyl Styrene in the Core and Tiecoat

The procedure of Comparative Example 1 was carried except thatIntermediate Example 4 core (185.2 g) was used; and ME 1 monomers werestyrene (60.0 g), MMA (44.4 g), MAA (9.6 g), t-BuSty (6.4 g). The finallatex had a solids content of 28.4%, a pH of 8.85, and a particle sizeof 403 nm. The dry density of this polymer was calculated to be 0.543g/cc. Low RH S/Mil was 1.40, High RH S/Mil was 1.33, and % Collapse was5%.

Comparative Example 2—Preparation of Aqueous Dispersion of PolymerParticles with Styrene in the Core

The procedure of Comparative Example 1 was followed except that the core(188.1 g) was made as described in Intermediate Example 5. The finallatex had a solids content of 28.5%, a pH of 8.7, and a particle size of403 nm. The dry density of this polymer was calculated to be 0.539 g/cc.Low RH S/Mil was 1.40, High RH S/Mil was 1.19, and % Collapse was 15.0%.

Example 6—Preparation of Aqueous Dispersion of Polymer Particles withStyrene in the Core and CHMA in Tie-Coat

The procedure of Comparative Example 1 was carried out except thatIntermediate Example 5 core (186.3 g) was use; and ME 1 monomers werestyrene (60.0 g), MMA (38.4. g), MAA (9.6 g), and CHMA (12.0 g). Thefinal latex had a solids content of 28.4%, a pH of 8.5, and a particlesize of 405 nm. The dry density of this polymer was calculated to be0.536 g/cc. Low RH S/Mil was 1.38, High RH S/Mil was 1.33, and %Collapse was 3.5%.

Example 7—Preparation of Aqueous Dispersion of Polymer Particles witht-Butyl Methacrylate in the Core and the Tie-Coat

The procedure of Comparative Example 1 was carried out except thatIntermediate Example 6 core (189.9 g) was used; and ME 1 monomers werestyrene (60.0 g), MMA (38.4. g), MAA (9.6 g), and t-BuMA (12.0 g). Thefinal latex had a solids content of 28.6%, a pH of 8.5, and a particlesize of 437 nm. The dry density of this polymer was calculated to be0.543 g/cc. Low RH S/Mil was 1.35, High RH S/Mil was 1.25, and %Collapse was 7.5%.

Example 8—Preparation of Aqueous Dispersion of Polymer Particles witht-Butyl Methacrylate in the Core and Cyclohexyl Methacrylate in theTie-Coat

The procedure of Comparative Example 1 was carried out except thatIntermediate Example 6 core (189.9 g) was used; and ME 1 monomers werestyrene (60.0 g), MMA (44.4. g), MAA (9.6 g), and CHMA (6.0 g). Thefinal latex had a solids content of 28.6%, a pH of 8.4, and a particlesize of 424 nm. The dry density of this polymer was calculated to be0.531 g/cc. Low RH S/Mil was 1.40, High RH S/Mil was 1.31, and %Collapse was 6.5%.

Example 9—Preparation of Aqueous Dispersion of Polymer Particles witht-Butyl Methacrylate in the Core and Tie-Coat

The procedure of Comparative Example 1 was carried out except thatIntermediate Example 7 core (189.3 g) was used; and ME 1 monomers werestyrene (60.0 g), MMA (38.4. g), MAA (9.6 g), and t-BMA (12.0 g). Thefinal latex had a solids content of 28.8%, a pH of 8.5, and a particlesize of 427 nm. The dry density of this polymer was calculated to be0.569 g/cc. Low RH S/Mil was 1.29, High RH S/Mil was 1.22, and %Collapse was 5.5%.

Example 10—Preparation of Aqueous Dispersion of Polymer Particles withIsobornyl Methacrylate in the Core

The procedure of Comparative Example 1 was carried out except that thecore (186.3 g) was made as described in Intermediate Example 8. Thefinal latex had a solids content of 28.5%, a pH of 8.8, and a particlesize of 424 nm. The dry density of this polymer was calculated to be0.540 g/cc. Low RH S/Mil was 1.31, High RH S/Mil was 1.21 and % Collapsewas 7.5%.

Example 11—Preparation of Aqueous Dispersion of Polymer Particles withIsobornyl Methacrylate in the Tie-Coat

The procedure of Comparative Example 1 was carried out except thatIntermediate Example 1 core (188.7 g) was used; and ME 1 monomers werestyrene (60.0 g), MMA (38.4. g), MAA (9.6 g), and IBOMA (12.0 g). Thefinal latex had a solids content of 28.6%, a pH of 8.9, and a particlesize of 413 nm. The dry density of this polymer was calculated to be0.539 g/cc. Low RH S/Mil was 1.27, High RH S/Mil was 1.23, and %Collapse was 3.0%.

The data demonstrate that dispersions of multistage polymer particleshaving a dry bulk density of less than 0.55 g/cc can be prepared with acollapse resistance of less than 10%, which is considered acceptable inthe field of opaque polymers.

1. A composition comprising an aqueous dispersion of multistage polymerparticles comprising a first and a second phase, wherein: a) the firstphase comprises, based on the weight of the first phase, from 25 to 60weight percent structural units of a carboxylic acid monomer or a saltthereof, from 40 to 75 weight percent structural units of a nonionicethylenically unsaturated monomer, and up to a total of 15 weightpercent structural units of styrene and a high T_(g) hydrophobicmonomer; and wherein b) the second phase comprises a mixture of firstand second polymers, wherein 1) the first polymer comprises, based onthe weight of the first polymer, i) from 5 to 15 weight percentstructural units of a carboxylic acid monomer or a salt thereof; ii)from 45 to 55 weight percent structural units of styrene; and iii) from37 to 47 weight percent structural units of methyl methacrylate or thehigh T_(g) hydrophobic monomer or a combination thereof; and 2) thesecond polymer comprises, based on the weight of the second polymer, i)from 80 to 99.9 weight percent structural units of styrene; and ii) from0.1 to 0.5 weight percent structural units of a multiethylenicallyunsaturated monomer; wherein the weight-to-weight ratio of the firstphase to the second phase is in the range of from 1:9 to 1:20; andwherein the weight-to-weight ratio of the first polymer of the secondphase to the second polymer of the second phase is in the range of from1:3 to 1:8; with the proviso that the first polymer and the first phaseof the second polymer together comprise, based on the weight of thefirst polymer and the first phase of the second polymer, from 2 to 15weight percent structural units of the high T_(g) hydrophobic monomer,which is one or more monomers selected from the group consisting ofcyclohexyl methacrylate, isobornyl methacrylate, 4-t-butyl methacrylate,t-butylstyrene, and n-butyl methacrylate.
 2. The composition of claim 1wherein the weight-to-weight ratio of the first phase to the secondphase is in the range of from 1:11 to 1:18; and wherein theweight-to-weight ratio of the first polymer of the second phase to thesecond polymer of the second phase is in the range of from 1:4 to 1:7.3. The composition of claim 2 wherein a) the first phase comprises,based on the weight of the first phase, from 25 to 50 weight percent ofstructural units of a salt of a carboxylic acid monomer, and from 40 to70 weight percent structural units of a nonionic ethylenicallyunsaturated monomer; and from 5 to 10 weight percent structural units ofthe high T_(g) hydrophobic monomer; wherein b) the second phase firstpolymer comprises, based on the weight of the second phase firstpolymer, from 6 to 12 weight percent structural units of a salt of acarboxylic acid monomer; and wherein c) the second phase second polymercomprises, based on the weight of the second phase second polymer, from83 to 91 weight percent structural units of styrene; from 8 to 12 weightpercent structural units of methyl methacrylate; from 0.5 to 4 weightpercent structural units of an alkali metal salt of methacrylic acid ofacrylic acid; and from 0.1 to 0.4 weight percent structural units of amultiethylenically unsaturated monomer.
 4. The composition of claim 2wherein a) the first phase comprises, based on the weight of the firstphase, from 30 to 50 weight percent of structural units of a salt of acarboxylic acid monomer, and from 50 to 70 weight percent structuralunits of a nonionic ethylenically unsaturated monomer; wherein b) thesecond phase first polymer comprises, based on the weight of the secondphase first polymer, from 6 to 12 weight percent structural units of asalt of a carboxylic acid monomer and from 5 to 15 weight percentstructural units of the high T_(g) hydrophobic monomer; and wherein c)the second phase second polymer comprises, based on the weight of thesecond phase second polymer, from 83 to 91 weight percent structuralunits of styrene; from 8 to 12 weight percent structural units of methylmethacrylate; from 0.5 to 4 weight percent structural units of an alkalimetal salt of methacrylic acid of acrylic acid; and from 0.1 to 0.4weight percent structural units of a multiethylenically unsaturatedmonomer.
 5. The composition of claim 2 wherein a) the first phasecomprises, based on the weight of the first phase, from 25 to 50 weightpercent of structural units of a salt of a carboxylic acid monomer, andfrom 40 to 70 weight percent structural units of a nonionicethylenically unsaturated monomer, and from 5 to 10 weight percentstructural units of the high T_(g) hydrophobic monomer; wherein b) thesecond phase first polymer comprises, based on the weight of the secondphase first polymer, from 6 to 12 weight percent structural units of asalt of a carboxylic acid monomer and from 5 to 15 weight percentstructural units of the high T_(g) hydrophobic monomer; wherein c) thesecond phase second polymer comprises, based on the weight of the secondphase second polymer, from 83 to 91 weight percent structural units ofstyrene; from 8 to 12 weight percent structural units of methylmethacrylate; from 0.5 to 4 weight percent structural units of an alkalimetal salt of methacrylic acid of acrylic acid; and from 0.1 to 0.4weight percent structural units of a multiethylenically unsaturatedmonomer.
 6. The composition of claim 5 wherein a) the second phasesecond polymer is a mixture of polymers comprising second phase secondpolymer A and second phase second polymer B, wherein b) second phasesecond polymer A comprises, based on the weight of second phase secondpolymer A, from 78 to 86.5 weight percent structural units of styrene,from 12 to 18.5 weight percent structural units of methyl methacrylate,from 1 to 4 weight percent structural units of methacrylic acid oracrylic acid, and from 0.1 to 0.4 weight percent of a multiethylenicallyunsaturated monomer; wherein c) second phase second polymer B comprises,based on the weight of second phase second polymer B, at least 98 weightpercent structural units of styrene, and wherein d) the weight-to-weightratio of second phase second polymer A to second phase second polymerBis in the range of from 3:1 to 6:1.
 7. The composition of claim 2wherein the second phase comprises less than 1 weight percent structuralunits of acrylonitrile, methacrylonitrile, acrylamide, andmethacrylamide, based on the weight of the second phase.
 8. Thecomposition of claim 7 wherein the second phase comprises less than 0.1weight percent structural units of acrylonitrile, methacrylonitrile,acrylamide, and methacrylamide, based on the weight of the second phase.9. The composition of claim 1 which further comprises a rheologymodifier, a binder, TiO₂, and at least one additive selected from thegroups consisting of surfactants, defoamers, biocides, dispersants,coalescents, and neutralizing agents.